Fig. 11. Original element concentrations C^O in mafic gabbro protoliths in comparison to altered element concentrations C^A in replaced rocks that are now garnet-sillimanite-cordierite gneisses (after Grant, 1986). Major oxide abundances are in wt%; trace elements are in ppm. Some values are proportionally scaled. Solid line = constant volume reference frame; short-dashed line = constant mass reference frame. Elements plotting to the left of any reference isocon represent gains; those to the right represent losses. Data points should be connected with a line to the origin (zero point). What is significant for the scaled values is not the distance of the data points from the lines representing constant mass or constant volume nor the distance from the origin but the angles or deviations of the lines to the data points relative to the central dashed or solid lines.